The present invention relates to an image reading device advantageously applicable to a copier or similar image forming apparatus.
In a digital copier, for example, a scanner scans a document to generate a corresponding image signal. The electric signal is transformed to digital data, subjected to predetermined image processing, and then sent to a printer incorporating a laser diode or similar light emitting device. The printer optically modulates the light emitting device on the basis of the digital image data. As a result, a light beam issuing from the light emitting device scans a photoconductive element to reproduce the document image thereon.
In an image forming apparatus of the type including a laser diode, for example, a laser beam issuing from the laser diode is steered by a polygonal mirror which is rotated at a high speed by a polygon motor. In practice, it is difficult to maintain the rotation speed of the polygon motor constant. It is, therefore, a common practice to control a document read timing by locating a beam sensor at a position where the laser beam starts scanning the photoconductive element. Specifically, the beam sensor generates a line synchronizing signal (LSYNC) synchronous to the rotation of the polygon motor and delivers this signal to the scanner. Since a substantial period of time is necessary for the polygon motor to reach a steady rotation speed after the start of the drive, it has been customary to continuously rotate the polygon motor at a constant speed even when the image forming apparatus is in a standby state. More specifically, the polygon motor is continuously driven so long as the power switch of the apparatus is turned on. This not only degrades the durability of the polygon motor but also generates noise.
Today, a multiplex digital copier configuration is attracting increasing attention in which an external line is connected to a digital copier to effect facsimile transmission and reception or a personal computer is connected to a digital copier to use it as an image data input and output terminal. In this kind of application, even when the copier does not have to perform a copying operation, e.g., when facsimile data is to be transmitted, the copier has to wait until the polygon motor reaches the steady rotation speed after the turn-on of the power switch and then read a document image in response to the line synchronizing signal from the beam sensor. This wastes time and, in addition, drives the polygon motor hard.
In the light of this, Japanese Patent Publication No. 6270/1990, for example, proposes an improved image reading device using signal generating means which generates, when an external synchronizing signal is not available with an external apparatus, a false synchronizing signal by counting pulses of a clock synchronous to an image signal from reading elements. The signal generating means is provided independently of inputting means for inputting the external synchronizing signal. A drive signal for driving the reading elements is generated in response to the external synchronizing signal or the false synchronizing signal. Hence, even when the external synchronizing signal from the external apparatus is not inputted, the reading elements can be desirably driven on the basis of the false synchronizing signal. Stated another way, even before the polygon motor starts up or when the printer is out of operation, the image reading device can read a document. However, the problem with this type of image reading device is that reading errors occur while the scanner reads a document, and it is difficult to match the device to the external apparatus with respect to the synchronizing signal.